1. Field of the Invention
The present invention relates generally to a novel adhesive composition, and more particularly to an adhesive composition or compound of fluorine-containing epoxy resin which has improved properties before and after curing and which can be controllably hardened or cured to form a cured product having a refractive index variable within the defined range in compliance with the desired design value. More specifically, it relates to an adhesive composition which may be used for the production of various optical members or parts for the applications in a communication system utilizing optical fibers.
2. Prior Art
Epoxy resins have hitherto been used for wide appliances as effective adhesives. The cured products of the adhesives containing epoxy resins are varied in their refractive indices, optical losses, adhesive strengths, flexibility and heat resisting properties extensively depending on the used epoxy resin and the combined hardeners or curing agents.
Amongst the known epoxy resins, the bisphenol-A type epoxy resins were predominantly used to form cured compositions having excellent adhesive and heat-resisting properties in combination with proper fillers and/or coupling agents. In order to further improve the flexibility of the cured products, other type epoxy resin adhesives containing epoxy resins derived from polyglycols have been developed. On the other hand, an adhesive composition added with a low viscosity diluent has been developed to lower the viscosity of the composition prior to the curing step and to facilitate defoaming after the addition of a hardener. Also developed was an adhesive composition in which an amine base hardener was used for lowering the curing temperature or for decreasing the time required for curing.
However, these known adhesive compositions have disadvantages that they have too high refractive indices not to match with those of optical members made of quartz fibers or a glass having a low refractive index, such as the BK7 glass (Borosilicate Crown glass mainly composed of SiO.sub.2, Ba.sub.2 O.sub.3, Na.sub.2 O and K.sub.2 O) or KZF.sub.2 glass (Antimony Flint glass mainly composed of SiO.sub.2, Ba.sub.2 O.sub.3, Na.sub.2 O and Sb.sub.2 O.sub.3) having low refractive indices to make it impossible for matching the refractive indices thereof when they are used as adhesives for adhering such optical members together. If the optical members are joined together by the use of any one of the aforementioned known adhesives, the reflection of the optical signal at the extremities of the optical members is increased to the extent that the strength of the signal is lowered too much to induce unstable operation of a light emitting element (e.g. a laser diode) disposed adjacent thereof. The adhesive compositions containing aforementioned bisphenol-A type epoxy resins have a disadvantage that the viscosities thereof in the uncured condition are too high.
Moreover, it is extremely difficult to suppress the optical transmission loss when a filler or binder is added to any one of the aforementioned resin compositions, and it also caused the reduction in flexibility or elasticity of the resultant cured composition. On the other hand, the heat-resisting property of the cured composition might be adversely affected by the addition of a low viscosity diluent.
It has been also known in the art that the adhesives containing epoxy resins derived from polyglycols are inferior in resistance to heat and low in adhesive strength. Particularly, it has been extremely difficult to prepare an adhesive composition excellent both in heat-resisting property and in flexibility by the use of the known epoxy resins.
Furthermore, when an adhesive composition is incorporated in an optical passage, scattering and/or reflection of the transmitted or travelling lights are promoted, leading to increase in optical loss, if foams are left in the cured resin composition. In order to minimize such optical loss, the resin composition must be defoamed completely after the step of adding and mixing with the curing agent or hardener. However, the conventional resin compositions can be hardly defoamed within the pot life time thereof after the resin systems are mixed with the curing agents or hardeners. It was, therefore, requisite and indispensable to mix the compositions in a special equipment so as not to form small foams or bubbles at the mixing step. It has been known to mix the resin composition in vacuum to obviate formation of air bubbles which are often left in the mixed composition to be incorporated in an optical passage. However, such process is cumbersome and requires the use of expensive devices.
In the art of manufacturing optical members, it has been known to use a so-called silicone oil, i.e. oily polysiloxane, to match the refractive indices of the joined parts. For instance, a silicone oil is used for joining optical fiber elements for the purpose of matching the optical indices. However, such a silicone oil is a liquid having some fluidity, and hence a container for preventing the used silicone oil from flowing out of the applied portion must be used to seal the matched portion. However, such a method adopted in the prior art for the matching of refractive indices includes complicated and troublesome operations with low yield. Moreover, the durability and/or reliability of the products processed through the method could not be insured.
When a laser diode is used and attached to one end of an optical fiber system, the reflected light enters into the laser diode to generate noises in the laser diode. It has been a common practice to deliberately dislocate the end of the optical fiber system relative to the laser diode thereby to differ the angle of incidence a little from zero, in other words, to couple the optical fiber system with the laser diode in a slightly dismatching interrelation for preventing the light reflected at the end face of the optical fiber system from returning back in the laser diode. As a result, it is impossible to maximize the transmitted light intensity by the use of the known adhesive composition. Accordingly, there is a demand for an improved adhesive composition having an extremely low reflectivity and well adapted for matching of the refractive index.
To solve the aforementioned problem, it has hitherto been proposed the use of an adhesive composition which may be adapted for the matching in refractive index in optical systems. However, almost all of the known adhesive compositions have refractive indices (n.sub.D.sup.23) ranging within 1.54 to 1.60, which are relatively high when comparing to those of the quartz glass (n.sub.D.sup.23 =0.46) and the BK7 glass (n.sub.D.sup.23 =1.5164). The measurement of refractive index is carried out at 23.degree. C. Therefore, when such a known adhesive composition is used in an optical system wherein a number of optical members having relatively low refractive indices in used or applied, the optical loss at the transmission interface is increased.due to increase in quantity of reflected light entering into the input side.
Further disadvantages of the known adhesive compositions are that they are unsatisfactory in their wettabilities to the optical members, and they are inferior in percent transmission (transmission factor), adhesive strength and resistance to heat. It is, thus, required to provide an adhesive composition which satisfies all of the aforementioned requirements for matching the refractive indices.
Recently, it is contemplated to establish an optical communication service network. To realize such contemplation, it is necessary to develop optical members used in a laser diode module, APD module (avalanche photodiode module) and an optical coupler. In assembly of the optical members, binders including solders or adhesives, are used at close positions in some cases. Although either one of a solder or an adhesive may be used for the purpose other than the fixation of the optical passages, a transparent adhesive must be used for the fixation in the optical passage. For this reason, development of an improved adhesive composition having high transparency is demanded.
The adhesive composition which may be used for the fixation purpose in an optical passage should have the following properties:
(1) Being improved in matching of refractive index with those of the optical members to be united; PA1 (2) To resist the heat applied at the soldering step for assembling the optical members; PA1 (3) Change in refractive index with the lapse of time thereof being as small as possible; and PA1 (4) Having satisfactory workability, i.e. being easily handled in the operational steps.
In order to shorten the time required for assembling the optical members, it is within the common technical knowledge of a person having ordinary skill in the art that a solder is used in place of the adhesive. In the practice of such alternation, the ferrule fixing the glass fiber with adhesive is fixed to a ferrule holder by soldering. One of commercially available adhesives was empirically used to join the end face of glass fiber system to the glass with anti-reflection layer. Then, the joined glass fiber system was subjected to soldering for fixing the ferrule to the ferrule holder. Whereupon, the adhesive at the joined interface became foamed due to the heat at the soldering step. By other experiments, it has been found that the refractive indices and/or other properties of the cured compositions are changed after the optical member bound by certain commercially available adhesives are assembled.